The Stirling or hot gas engine cycle is well known from in the art. A two-cylinder Stirling engine is described in U.S. Pat. Nos. 3,984,983 and 3,999,388 while a four cylinder engine is further described in U.S. Pat. Nos. 3,914,940 and 4,474,003. The Stirling engine is durable, clean burning, and exhibits relatively high efficiency when compared to the more conventional internal combustion engine. The Stirling engine, however, is relatively slow to respond to changes in power demands and thus difficult to adapt for use in motor vehicles where engine acceleration and deceleration must be rapid. Recently, efforts have been undertaken to improve the response time of the Stirling engine sot that it might be better suited for use in motor vehicles.
Power control of Stirling engines by variation of mean pressure is known to provide a very good part-load efficiency, and it does not require variable kinematics (as variable stroke and phase do). These qualities make mean pressure variation highly desirable for uses such as motive power where much of the operation is part-load and size and fast transient capability are very important. Unfortunately, with a fixed inventory of working gas available in engine and storage vessel, a pump is required to move gas at least one way (in or out) between engine and storage. In practice, this pump takes the form of a positive-displacement gas compressor of rapid response capability and often variable capacity. Such units are unavoidably expensive, bulky, and less reliable than required.
U.S. Pat. Nos. 4,601,171; 4,601,172; 4,612,769; and 4,655,036 (all assigned to the assignee of the present application) disclose different aspects of a control scheme where 2 or more storage bottles (tanks) are maintained at different pressures so that the pressure ratio across the compressor is minimized. Thus each of these patents describes a control system utilizing a compressor.
FIG. 2 shows a known one tank, non-compressor system. Such a system can modulate pressure and power over a limited range depending on engine pressure and tank capacity. In such a one tank system, however, when tank volume is infinite or pressure is otherwise held constant at P.sub.1, then the minimum available power a large fraction of maximum power. The situation is worse if flow into and out of a finite volume tank affects its pressure level.